Polyolefins have poor compatibility with a polyamide matrix. As a result, it is difficult to obtain blends which are both processable and practical and various expedients have been adopted in an attempt to solve this problem.
British Patent 998439 (German 1,241,606) teaches achieving high impact resistance by mixing two polyolefins into the rigid polyamides. In order to overcome the incompatibility problem, the olefins have unsaturated carboxylic acids grafted thereon.
Swiss Patent 649566 seeks to solve the same problem by introducing polyolefins based upon ethylene, propylene, and 1,4-hexadiene or 2,5-norbornadiene. These materials are activated by the introduction of .alpha., .beta. unsaturated dicarboxylic acids, anhydrides, or esters. Thus, the high bending E-modulus of the polyamide is modified by the unsaturated additives. Molded shapes prepared from such compositions have good impact and notched bar impact properties and high ductility. However, they are not sufficiently rigid, nor are they dimensionally stable. In other words, they have a great tendency to creep, which makes them unsuitable for many uses.
Another approach to the same problem is the introduction of core/shell polymers or grafted copolyolefins into rigid amorphous copolyamides. Such compositions are disclosed in U.S. Pat. Nos. 4,410,661 and 4,536,541. However, mixtures of the foregoing type exhibit substantial and undesirable shrinkage at elevated temperatures, even after only a short period of time. Therefore, shapes produced from such compositions are unsuitable for any use wherein exposure to elevated temperatures is involved.
A narrower improvement is shown in German 3,436,362, wherein rigid polyamides derived from terephthalic acid, isophthalic acid and alkanediamines form the basic material. To this is added copolymers of ethylene and/or acrylates to which carboxyl groups are grafted.
European Patent 27,198 teaches the addition of a core/shell polymer as a modifier of impact resistance. Acrylic acid derivatives are grafted onto a polybutadiene or butadiene/styrene core. This improves the impact resistance, but results in a substantial loss in stiffness. The patent teaches compensating for this loss by the addition of glass fibers.
U.S. Pat. No. 4,180,494 teaches the impact-resistance modifiers of the present invention, and German 3,339,000 teaches the use of similar core/shell polymers in conjunction with polyamides. However, molded shapes of this character are usually unsatisfactory, as they are not dimensionally stable. In particular, they tend to creep and to be highly susceptible to shrinkage under heat.
British Patent 1,069,176 seeks high thermal and dimensional stability for molded shapes by the addition of cross-linking diepoxides.
British 1,376,537 and German 2,144,687 introduce epoxide resins into plastomeric materials. These are substantially dimensionally stable and rigid; however, their impact-resistance is insufficient. Moreover, the presence of a high concentration of fillers and reinforcing agents produces a high density which, in many cases, is to be avoided.
Thus, there is a substantial need for polymeric materials which are impact-resistant, rigid, and dimensionally stable. In addition, such materials should be capable of simple injection molding. Such materials are used as casings and other protective parts in machines, vehicle bodies, etc.